Pulsars are the dense, rapidly spinning remains of stars much more massive than the Sun. To really get a pulsar revolving quickly, it needs a companion star: matter stripped from the partner falls onto the pulsar, speeding it up until it can rotate hundreds of times every second. Astronomers discovered these millisecond pulsars by their radio emissions, but many of them are also very strong gamma ray sources.

Astronomers have now used the Fermi Gamma-Ray Space Telescope to identify a "black widow" pulsar that's stripping mass off a close companion star while simultaneously evaporating it by emitting intense radiation. It's having these dramatic effects because the pulsar and its companion orbit each other so closely that they complete an orbit once every 93 minutes, making this the tightest black widow binary yet discovered.

Many radio telescopes are well suited to hunting for pulsars, since they can scan large parts of the sky and pick out the periodic flashes of light these bodies emit. Gamma ray observations are harder for a number of reasons, not least of which is that Earth's atmosphere is (thankfully) opaque to gamma radiation (which means we need space-based telescopes). The only existing gamma ray instrument with the sensitivity to spot millisecond pulsars is the Large Area Telescope (LAT) on the orbiting Fermi observatory.

To find pulsars in the maps provided by LAT, Holger Pletsch and colleagues sifted through nearly four years of data using a "blind search," sorting through all the data without targeting a specific source. That method would allow them to identify sources that might be faint or otherwise difficult to spot, but it was also computationally intensive. They began by taking 12-day chunks of time from four years of LAT data, looking for periodic fluctuations.

The hunt was complicated by the nature of millisecond pulsars themselves: they fluctuate on a very short time scale, and the specific physical parameters—rate of pulsation, amount of change in that rate, and the properties of the binary system—cannot be known in advance. In other words, it's not as easy as looking for a flashing light on a dark night: it's more like scanning grainy film footage one frame at a time, looking for the same flashing light.

Enlarge/ The location of the gamma ray millisecond pulsar in the Fermi all-sky map. The bright band across the center of the image is the Milky Way; brighter colors indicate higher gamma ray intensity.

NASA/DOE/Fermi LAT Collaboration/AEI

Despite these difficulties, the researchers found a clear candidate, now known as PSR J1311-3430. They measured the pulsar's rotational period to be about 2.5 milliseconds, meaning it spins 400 times every second. In fact, the pulsar corresponded to a bright gamma ray source (2FGL J1311-3430 for you gamma ray enthusiasts) seen in the Fermi all-sky map. Since this object (along with many others) had no corresponding radio emission, it was considered an unidentified source.

Using the Fermi data and optical observations, the astronomers determined the companion star was extremely close to the pulsar, with the binary completing their mutual orbit in 93 minutes. Such a tight orbit means that not only is the pulsar accreting mass from its companion to increase its spin rate, but the intense radiation from the pulsar is blasting away the outer layers of the star. The pulsar heated up its companion so much that the low-mass star has inflated to a large size. This type of system is known as a black widow, because the pulsar is devouring its mate.

The particulars of the system also explained why there was no identifiable radio source at the same location. The wild winds of material stripped from the companion star would tend to scatter and obscure radio emission, so even if the pulsar was emitting radio pulses (a very likely scenario), they wouldn't be coherent as seen from Earth.

All of this leads to hope that other bright, unidentified gamma ray sources might also be millisecond pulsars. While the hunting process is intensive and time-consuming, the researchers expressed optimism, not least since many unknown objects are known and can provide immediate targets rather than using blind searches. Furthermore, such binary systems provide tests of the most extreme physics, including gravitational radiation and the properties of the pulsars themselves.

23 Reader Comments

It's having these dramatic effects because the pulsar and its companion orbit each other so closely that they complete an orbit once every 93 minutes, making this the tightest black widow binary yet discovered.

I'm having trouble with scale here: how big are these objects, and how far apart?

"The wild winds of material stripped from the companion star would tend to scatter and obscure radio emission, so even if the pulsar was emitting radio pulses (a very likely scenario), they wouldn't be coherent as seen from Earth."

This confuses me, since apparently the gamma rays get through those "wild winds" just fine (that's what they detected), and gamma rays and radio waves are both electromagnetic radiation. But gamma rays are much, much higher frequency than radio waves, and thus would normally have much more difficulty penetrating intervening material. You would expect the radio waves to get through preferentially compared to gamma rays (just like gamma rays are usually absorbed by our atmosphere, and radio waves not so much), rather than the other way around.

Is it just the difference in intensity that makes us able to detect the gamma rays and not the radio waves? Or is it the fact that the gamma rays are higher energy and therefore easier to detect?

From my understanding it's the much higher energies with Gamma Rays that enables their relatively easy detection, even across cosmological scales. The energies associated with them are mind boggling in some instances.

It's a crying shame to be Earth bound when we're just now finding all these wonderful things in the cosmos. I'd give almost anything to see something like that live with my very own eyes. Albeit briefly, for I'm certain I'd be vaporized myself in short order.

The fact that the pulsar is so significantly larger than our Sun and it's spinning at 2.5ms/rotation boggles my mind that something that large can have such a massive rotational speed. Eventually, we get to a scale where our mind just can't quite comprehend what we're reading.

This sounds like a plotline from a really bad sci-fi flick. It's incredible that it's real. Imagine being able to observe it happening.

A quick back-of-the-envelope calculation assuming a 35 km diameter pulsar and a rotation rate of 2.5 milliseconds, would mean that the surface of the pulsar is traveling at about 15% of the speed of light. Crazy.

Nah, I'm just being snarky (I understand that to be the prime function of the intarwebs), I get it. (We know SOMETHING is there, just not what it is.) But I get the feeling that it could maybe stand to have an edit for clarity?

. o O (That back-of-the-envelope surface speed calculation comment is full of awesome, and I would like to subscribe to that poster's newsletter.)

Dang. A relativistic spin on an object larger than our sun, and it orbits it's companion every hour and a half? Amazing. The speeds being talked about here are ahem, mind boggeling. Think of the tidal forces between those stars. If only one day we could get a telescope a few light-years away from an event like that and take high res photos.

A quick back-of-the-envelope calculation assuming a 35 km diameter pulsar and a rotation rate of 2.5 milliseconds, would mean that the surface of the pulsar is traveling at about 15% of the speed of light. Crazy.

Thanks much. That's just the calculation I wanted to perform by obtaining the diameter.

A quick back-of-the-envelope calculation assuming a 35 km diameter pulsar and a rotation rate of 2.5 milliseconds, would mean that the surface of the pulsar is traveling at about 15% of the speed of light. Crazy.

Closer to 14.5% of light - but crazy nonetheless. It's still speeding up as it consumes its companion!

1. the neutron star isn't larger than our sun. It's more massive, but MUCH more compact.2. I believe the record for neutron star spin speed is 1/3 light speed.

When rotational speeds are measured in reasonable fractions of c, what effect does relativity have upon the rotating body?

What's the ratio of outward acceleration on the surface from rotation compared to the inward acceleration due to gravity?

Shouldn't there be some upper bound to the rotational speed in comparison to its radius and mass? I can imagine these stars spinning so fast that they fly apart. Or alternatively it'd eject as much mass due to rotation at the same rate as it would be taking in mass from a companion star.

Fairly close, but definitely less. It's a bit interesting also that the escape velocity and surface velocity are so close to each other, within an order of magnitude. Enough of a coincidence that it might make you think a bit about the possibility that it might have actually flung off nuclear matter at an earlier age when the the rotation rate was faster (if it is spinning down because of gravity wave radiation rather than still spinning up because of accretion).

When spinning that fast, wouldn't the outer layer overcome the force of gravity and be slung out into space? If not, how fast would it have to spin for this to happen?

According to http://www.nrao.edu/pr/2006/mspulsar/ , the pulsar spinning at 716 Hz can have a diameter no greater than 20 miles, otherwise matter starts getting flung. That corresponds to the .24c equatorial speed mentioned earlier. Dunno how they calculate escape velocity of a huge spinning ball of neutrons, but I bet there's math involved.